The following discussion is not to be taken as an admission of relevant prior art.
Printable transparent conductive films are characterised by having high transparency, low sheet resistance, and stability at high current flow. Printable conductors have been used in a wide variety of optoelectronic devices (e.g. solar cells, solid state lighting, and touch screen displays). Transparent conductors are often defined as thin conductive films coated on high-transmittance surfaces or substrates. Optoelectronic devices require the electrodes to be transparent, typically using thin films of indium tin oxide (ITO) vacuum deposited onto substrates. However, the high expense, proneness to defects and fragility of ITO have led to a sharp increase in interest of alternative transparent conductors. Moreover, the process of vacuum deposition is not conducive to forming patterns and circuits, typically requiring expensive patterning processes such as photolithography. An existing method for creating patterned electrodes using vacuum deposited ITO include laser patterning, however this technique is limited by the complexity of the pattern, with more complex patterns requiring long processing times. This makes the laser patterning of ITO less economical for the production of complex transparent electrodes.
One answer to this has been to print patterns of conductive inks containing electrically conductive polymers such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) mixed with various polymeric additives to aid processing. However, conductive polymers are inherently coloured and possess low long-term chemical stability which are disadvantageous for application in optoelectronic devices. An alternative method is to disperse a conductive material (such as metal nanoparticles) in a solvent along with polymeric additives.
The use of a polymeric additive in the conductive ink mixture is exemplified in Chinese Patent Publication No. 2011/10444509 entitled “Preparation method of haze adjustable flexible transparent conductive film” and U.S. Pat. No. 8,815,126 entitled “Method and composition for screen printing of conductive features”. These documents describe the use of a polymeric additive as an insoluble resin that acts as both a viscosity modifier to aid printability and as a binder to adhere the conductive material to a substrate. The problem with using polymeric additives is that they will also coat the conductive particles in the ink, decreasing the amount of contacts between conductive particles and increasing the percolation threshold of the mixture. To achieve similar conductivity typically requires a much larger amount of conductive material than when polymeric additives are not present, which is often prohibitively expensive and more importantly the higher loading of conductive materials decreases the transparency of the electrode. Such obstacles have thus far prevented the establishment of a permanent replacement of ITO as transparent electrode material.
Efforts to create a transparent electrode using copper nanowires can be found in, for example, U.S. Patent Publication No. 2012/0061124 to Cui et al., entitled “Electrodes and electrospun fibers”. This reference discloses an electrode composed of a network of copper nanofibers having good overall flexibility, a sheet resistance of 200 Ω/sq (Ohms per square) and a transmittance of around 96% for visible and near infrared (i.e., 300-1100 nanometers). This can be judged as better performance than commercial vacuum deposited ITO; however, copper nanostructured electrodes will quickly oxidize when exposed to moisture and oxygen in the atmosphere. Oxidation of the surface of the copper nanofibers will significantly degrade the electrical conductivity of these nanostructured networks. Additionally, those versed in the art will know that electrospinning of a conductive network is not suitable for the creation of patterned electrodes without several subsequent labour intensive processes. Consequently, there remains a need for a new flexible nanostructured transparent electrode design that can benefit from the advantages of using metallic nanostructures whilst also being resistant to rapid environmentally effected degeneration (e.g. reduced conductivity).
The application of conductive nanomaterials in the production of transparent electrodes has been disclosed in a number of forms, for instance, Patent Publication No. WO 2014/116738 to Allemand et al. entitled “Nanostructure transparent conductors having high thermal stability for ESD protection”. This document discloses the over-coating of a protective layer onto a pre-deposited network of metallic conductive material to prevent or limit degradation of conductivity caused by environmental exposure. However, this technique limits the application of the transparent electrode by forming an insulating coating on the top surface of the electrode. Some devices require top surface conductivity to allow interaction with electrically active materials/components placed at various points on the top surface of the electrodes, such as within passively-addressed optoelectronic devices.
A number of silver-based transparent conductive films containing silver nanowires exist but they typically involve a number of complicated processing steps, for instance involving extensive silver nanowire pre-coating of a shaped “stamp” which is then used to transfer a conductive network onto the final substrate to achieve the desired electrode shape. The printing of patterns of conductive nanomaterials to produce transparent electrodes is already known, for example, US Patent Publication No. 2007/0284557 to Gruner et al., entitled “Graphene film as transparent and electrically conducting material”. Such techniques as featured in this document include forming a uniform film of conductive material (e.g. graphene) by dispensing the supernatant of a graphene dispersion though a piece of filter paper and then transferring a pattern of the deposited film onto the final substrate by transfer printing using a PDMS stamp. The poor applicability of such a process would however present a number of problems for large patterns (limited by the size and ability to form a large uniform film) or different substrate materials (transferring a thin film onto a rough surface would provide significant problems) as well as expensive commercial scale production of such a labour intensive process.